The leaching of copper, nickel and zinc sulfide ores and concentrates under pressure with a solution of ammonium sulfate containing free ammonia (ammonium hydroxide) in the presence of oxygen is well known. Such a procedure is described for example in Forward U.S. Pat. Nos. 2,576,314, 2,726,934 and 2,822,263. Recently a process has been developed for leaching such sulfide ores and concentrates at only a few psig (pounds per square inch gauge pressure) with ammonium sulfate solution containing free ammonia and in an oxygen containing atmosphere, by vigorously agitating a slurry of the ore or concentrate in such solution at a temperature from 60.degree. to 100.degree. C. and vigorously recirculating the oxygen to beneath the surface of the slurry.
The basic reaction by which the metal sulfide mineral is brought into solution can be summarized as follows: EQU MS+4NH.sub.3 +20.sub.2 .fwdarw.M(NH.sub.3).sub.4 SO.sub.4
mainly, the sulfur of the dissolved mineral is oxidized to sulfate as the metal constituent is solubilized in the form of a metal-ammonium complex. When the pregnant leach solution is treated to recover the dissolved metal values (e.g., by chemical reduction with hydrogen on carbon monoxide, by electrochemical reduction in an electrowinning operation, or by treatment with an ion exchanger to extract the metal) the sulfate constituent of the complex remains in solution as ammonium sulfate. Economy requires that some provision be made to recover and reuse the gaseous ammonia content of this solution and to separate and discard accumulated sulfate. A well-known procedure for doing so is to treat the residual barren leach solution, after recovery of its metal values, by boiling with lime. Thereby the sulfate ion is precipitated as gypsum, which may be discarded, and the ammonium ion is released as gaseous ammonia which can easily be recovered.
Simple and effective as this lime boil procedure is, it is subject to the serious disadvantage that a large amount of heat is required for it. Indeed, in a complete cyclic process involving leaching, which includes metal recovery from the leach solution, a lime boil of sufficient barren leach solution to eliminate the amount of sulfate dissolved during leaching, and recycling to the leaching operation of the untreated portion of the barren leach solution plus the ammonia recovered in the lime boil, the greatest single energy-consumer is the lime boil operation.
An alternative to the lime boil is to evaporate water from enough of the barren leach solution to crystallize ammonium sulfate in an amount equivalent to the amount of sulfide oxidized to sulfate during leaching. This procedure may in some cases require less heat input than a lime boil, but it is economically practical only when an adequate market exists for the crystallized ammonium sulfate.
So far as we are aware, no other satisfactory alternative procedures for a lime boil operation or an ammonium sulfate crystallization operation have heretofore been proposed for eliminating sulfate dissolved during leaching or recovering the ammonia combined with it.